Guide to Predict Seasonal Rainfall of Batticaloa

INTRODUCTION

Batticaloa is one of the district suffers the most due to natural disasters. At the same time main economy of this district is Agriculture, primarily in the form of paddy. Due to the annual variation of the seasonal rainfall paddy cultivation failed to give the maximum yield.

Unexpected heavy rainfall causes floods and less rainfall results drought. This affect the cultivation as well as living standard of people in these areas. We science navigators were able to review the studies published so far regarding climate of Batticaloa. As a result we managed to formulate a prediction model for seasonal rainfall in Batticaloa. This was reviewed by Climatologist Prof Bjo¨ rn A. Malmgren, Professor Emeritus of Marine Geology.

We believe this will help farmers, administrators and other relevant official to plan agriculture activity and disaster management system in Batticaloa.

Predicting Seasonal rain in Batticaloa District

Monsoon system governing the Sri Lankan rainfall climate

The two principal monsoon seasons are the Southwest Monsoon (SWM; May through to September) and the Northeast Monsoon (NEM; December through to February). The two Inter Monsoon rainfall seasons, the First Intermonsoon (FIM; March through to April) and Second Intermonsoon (SIM; October through to November).

During the Intermonsoon seasons, convectional type rainfall and tropical depressions (mainly during the SIM) originating in the Bay of Bengal are predominant, and heavy rainfall spells within a short period of time are frequent during these seasons.

Monsoon systems influence Dry Zone (Batticaloa) rainfall

Batticaloa situated in Dry Zone of Sri Lanka.

All the above seasons can have an impact, but precipitation is primarily governed by the NEM and SIM. Trincomalee and Batticaloa from the eastern coastal region receive a mean > 262 mm/month, whereas stations Ratnapura and Badulla receive means of 164 mm/month and 196 mm/month respectively from the south central parts show the high NEM-related precipitation.

North-East Monsoon Rainfall

Contribution of SWM (South west Monsoon) to Batticaloa rainfall (during May through September)

Generally SWM has little contribution to Batticaloa rainfall.

Other Global climate phenomena that can affect the rainfall climate

ENSO events (El Niño, neutral, and La Niña)

Indian ocean dipole (positive, neutral and negative)

How ENSO events affects SWM (South west Monsoon) to Batticaloa rainfall (during May through September)

Batticaloa from the eastern part of Sri Lanka, and Jaffna, the most northerly station, show significantly larger precipitation during La Niña years than during El Niño and neutral years.

Except Badulla and Ratnapura, most of other locations that have been analyzed for precipitation pattern in Sri Lanka show pronounced differences between one and several of the ENSO events. All of the remaining stations show statistically significantly greater precipitation amounts during El Niño than during La Niña years.

For example, at some of these stations (Batticaloa, Diyatalawa, and Nuwara Eliya) the El Niño El Ni˜no years stand out above both the neutral and La Niña years.

These stations receive, on average, between 14 (Batticaloa) and 20 mm/month (Jaffna) more SWM rainfall during La Niña years than during El Niño years.

A recent and larger study did not find a clear coupling between the ENSO events and NEM and FIM season precipitation in Sri Lanka. However a few studies pointed at the existence of enhanced El Niño rainfall in extreme southern India and Sri Lanka during the NEM season. One study found less NEM season rainfall during La Niña conditions during August–October.

So in conclusion, El Niño conditions will not reduce the rainfall during NEM.

How the Indian Ocean Dipole (IOD) affects NEM

The wind pattern associated with the positive phase of the IOD suggests a transfer of moisture from the eastern

On the other hand, the negative phase of the IOD inhibits transport of moisture convergence towards India and Sri Lanka suppressing rainfall activity.

Link between extreme and deficient NEM and IOD

One of the important features of the Northeast Monsoon over the Indian subcontinent is the establishment of the northeasterly winds over the Bay of Bengal. Another feature is the weak westerly winds over the equatorial Indian Ocean.

Anomalous southeasterly/easterly winds prevail over the equatorial Indian Ocean from the Sumatra coast during excess monsoons. This is accompanied by northeasterly/easterly winds over the Bay of Bengal.

Anomalous southeasterly/easterly winds prevail over the equatorial Indian Ocean from the Sumatra coast during the positive phase of IOD.

Both these wind systems converge over the southern parts of India, in particular over the southeast peninsula. This flow will result in an anomalous moisture convergence over south India and Sri Lanka. This should result in excess rainfall.

When equatorial wind system flows westerly and wind over Bay of Bengal flows northwesterly NEM will become deficient. Both wind systems, from the central Indian Ocean and from the Bay of Bengal, converge over Sumatra, suggesting moisture transport towards Sumatra. This flow will inhibit rainfall activity over southern parts of India.

La Niña climate conditions bring more than average rain fall to Batticaloa during south-west monsoon (SWM) season (May to September) all other seasons get usual average rain fall.

El Niño climate conditions bring more than average rain fall to Batticaloa during second Intermonsoon (SIM) season (October and November). All other seasons get usual average rain fall. Neutral climate condition brings usual average rain fall to all seasons.

Positive Indian Ocean dipole brings more than average rain fall to SIM and NEM seasons. All other seasons get usual average rain fall. While negative IOD brings less than average rain fall to these seasons.

El Niño and positive IOD bring much more rain during SIM season.

Effects on rain fall depend on the severity of the climate condition. For example, strong El Niño brings more rain than weak El Niño during SIM season.